How can curves be converted to meshes?
The reason you can't use a "constructive modifier" on a curve is because a curve has to be defined by mathematical control points instead of free-floating vertices. You can visualize the decimate modifier in the viewport or render, but if you were to actually apply it you might create a shape that can't be represented by the control point equations, and then you would have broken mathematics.

The way to fix this is to convert your curve to a mesh. Select the tree and use AltC:



Here's the result:



Then any mesh-based modifiers will work fine.